'''Pyrometallurgy''' is a branch of extractive metallurgy It consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable metals Pyrometallurgical treatment may produce saleable products such as pure metals or intermediate compounds or alloys suitable as feed for further processing
Pyrometallurgical processes are generally grouped into one or more of the following categories:
- Drying
- Calcining
- Roasting
- Smelting
- Refining
Most pyrometallurgical processes require
energy input to sustain the
temperature at which the process takes
place The
energy is usually provided in the form of fossil fuel
combustion exothermic reaction of the
material or from electrical
heat When enough
material is present in the feed to sustain the process
temperature solely by
exothermic reaction (ie without the addition of fuel or electrical heat) the process is said to be "autogenous"
Drying
Drying is thermal removal of liquid moisture (not chemically bound) from a
material Drying is usually accomplished by contacting the moist solids with hot
combustion gases generated by burning fossil fuels In some cases
heat for
drying can be provided by hot air or
inert gas that has been indirectly heated The amount of
heat required for a given
drying operation corresponds to the
heat required to vaporize the liquid moisture the
heat required to raise the
temperature of the products (dry solids and
water vapor) to the final
drying temperature and
heat required to offset radiant
heat losses
Usually the
drying temperature is set at a nominal value above the
boiling point of
water often about 120°C In special cases such as in the
drying of certain water-soluble salts higher
drying temperatures are required In salt
drying the feed moisture is saturated with dissolved salts which alters the
boiling point and requires higher
drying temperatures
Drying of moist solids is carried out in several types of industrial dryers including rotary dryers fluidized bed dryers and flash dryers
Another type of
drying called
spray drying is carried out when the
material to be dried is completely dissolved in
aqueous solution The
solution is sprayed (usually through a specially designed nozzle) into a heated chamber and as the
water is evaporated solids crystallize The
water vapor is exhausted from the dryer and dry solids are collected usually in a conical section of the dryer Solid
material produced from a spray dryer often has special particle size and shape characteristics which may be controlled by the
concentration of dissolved
material in the
solution and the
design of the atomizing spray nozzle
Calcining
Calcining is
thermal decomposition of a
material Examples include decomposition of hydrates such as
ferric hydroxide to
ferric oxide and
water vapor or decomposition of
calcium carbonate to
calcium oxide and
carbon dioxide and or of
iron carbonate to
iron oxideCalcination processes are carried out in a variety of furnaces including shaft furnaces
rotary kilns and
fluidized bed reactorsRoasting
Roasting consists of thermal gas-solid reactions which can include oxidation reduction chlorination sulfation and pyrohydrolysis
The most common example of roasting is the oxidation of metal sulfide ores The metal sulfide is heated in the presence of air to a
temperature that allows the
oxygen in the air to react with the sulfide to form
sulfur dioxide gas and solid metal oxide The solid product from roasting is often called "calcine" In sulfide roasting if the
temperature and gas conditions are such that the sulfide feed is completely oxidized the process is known as "dead roasting" Sometimes as in the case of pre-treating reverberatory or electric
smelting furnace feed the roasting process is performed with less than the required amount of
oxygen to fully oxidize the feed In this case the process is called "partial roasting" because the
sulfur is only partially removed Finally if the
temperature and gas conditions are controlled such that the sulfides in the feed react to form metal sulfates instead of metal oxides the process is known as "sulfation roasting" Sometimes
temperature and gas conditions can be maintained such that a mixed sulfide feed (for instance a feed containing both
copper sulfide and
iron sulfide) reacts such that one metal forms a sulfate and the
other forms an oxide the process is known as "selective roasting" or "selective sulfation"
Smelting involves thermal reactions in which at least one product is a molten phase
Metal oxides can then be
smelted by heating with coke or
charcoal (forms of
carbon) a
reducing agent that liberates the
oxygen as carbon dioxide leaving a refined mineral Concern about the production of carbon dioxide is only a recent
worry following the identification of the enhanced greenhouse effect
Carbonate ores are also smelted with charcoal but are sometimes need to be calcined first
Other materials may need to be added as
flux aiding the
melting of the oxide ores and assisting in the formation of a
slag as the
flux reacts with impurities such as
silicon compounds
Smelting usually takes
place at a
temperature above the
melting point of the metal but processes vary considerably according to the ore involved and
other matters
Refining
Refining is the removal of impurities from materials by a thermal process This covers a wide range of processes involving different kinds of
furnace or
other plant
The term 'refining' can also refer to certain
electolytic processes Accordingly some kinds of pyrometallurgical refining are referred to as 'fire refining'
Environmental aspects
The gases produced by pyrometallurgical processes often present air
pollution problems In the early part of the 20th century gaseous
pollution from pyrometallurgical processes was largely uncontrolled For example waste gases from roasting near Queenstown Tasmania over the years killed off all the
vegetation which then allowed all the top soil to erode The
result was a dramatic and unnatural change to the surrounding terrain Dust or "fume" from pyrometallurgical processes presented serious hazards related to the health and safety for workers in, and residents near pyrometallurgical plants These metallurgical dusts often contained
arsenic cadmuim mercury
lead zinc and
other heavy metals At the same
time dust losses from metallurgical plants also represented loss of potentially valuable product Dust capture technologies were developed such as baghouses and electrostatic precipitators Gas treatment techniques were also developed such as wet gas scrubbing Additionally as in the case of
sulfur dioxide techniques were developed for production of sulfuric
acid from waste gases
See also
External links
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